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Abstract: In this paper, a digital holography based method is proposed to accurately measure the length and orientation of the needle-shaped particles in solution. The method involves recording of the hologram and numerical reconstructions (focusing) of the hologram at several depths. An image analysis routine is then used to determine the length, location and orientation of the particles from the reconstructed images without any a priori information about the orientation of particles. The performance of the method is verified using a single fiber with known size and orientation. Subsequently, the proposed method is applied to a suspension of fibers, where the length measurements are found to be in good agreement with the true values. This proposed technique can overcome the shortcoming of existing 2D imaging tools, which can provide only the projected lengths of randomly oriented particles. [Copyright &y& Elsevier]